Twist drill

ABSTRACT

The present invention provides a twist drill. A cone portion is provided at a front end of the operating portion, and an exterior surface of the operating portion is provided with a spiral flute for shunting cutting chips. The exterior surface of the cone portion is provided with a plurality of composite cutting blade groups which are sequentially enlarged in diameter from the front end to the rear end of the cone portion. The cone portion is provided with a top blade on the tip. In use, the top blade is used for positioning, and the cutting process is carried out by the top blade and the composite cutting blade groups.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/699,089, filed on Nov. 28, 2019, which is a continuation-in-part ofU.S. patent application Ser. No. 15/847,900 filed on Dec. 19, 2017, nowabandoned, which is a continuation-in-part of International PatentApplication No. PCT/CN2016/073767, now expired, filed on Feb. 14, 2016.The contents of the aforementioned application, including anyintervening amendments thereto, are incorporated herein by reference inits entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The application relates to metal cutting and metal cutting tools, moreparticularly, to a twist drill.

Description of the Related Art

FIGS. 1A-B are schematic diagrams of a conventional twist drill. FIG. 1Ais a front view of the twist drill; FIG. 1B is a top view of a drilltip; and FIG. 1C is a perspective view of a truncate twist drill. Twistdrills are commonly used in the places away from other metal cuttingmachines such as drill presses, and when a hand-held electric tool isused for the drilling operation, the drilling efficiency is greatlyconstrained due to limitations of arm strength of a worker, power of theelectric tool and the like. Thus, the existing twist drill is difficultto position the drill hole, and low in speed and efficiency.

FIG. 2 shows the cutting process of the twist drill in the prior art ofFIGS. 1A-C. When the twist drill with such a conventional structure isused for drilling, the processing of a metal cutting amount in acorresponding size is simultaneously completed by two symmetricallydistributed linear cutting edges, so that a relatively large power isrequired during drilling. Meanwhile, relatively large reaction forcesare also applied onto two cutting edges, so the cutting edges are easyto be damaged.

SUMMARY OF THE INVENTION

The present invention provides the following technical solutions tosolve the technical problems that a twist drill in the prior art isdifficult to position the drill hole, and low in speed and efficiency.

The present invention provides a twist drill, comprising: a shankportion and an operation portion connected to the shank portion; theoperation portion comprises a cone portion and a cylinder portion whichis fixedly connected to the cone portion; a spiral flute for shuntingcutting chips is provided on an exterior surface of the operationportion, and the spiral flute is wound from a front end of the coneportion to the cylinder portion.

A plurality of composite cutting blade groups are provided on theexterior surface of the cone portion, and diameters of the compositecutting blade groups sequentially increase from the front end to a rearend of the cone portion; and the composite cutting blade groups eachcomprise a first step surface and a second step surface which areadjacent.

A top blade is provided on a tip of the cone portion.

In some embodiments, top ends of flute sidewalls of the spiral fluteform a plurality of spiral lines.

An intersection of the first step surface and the flute sidewall of thespiral flute is a major cutting edge; an intersection of the second stepsurface and the flute sidewall of the spiral flute is a minor cuttingedge; and an intersection of the major cutting edge and the minorcutting edge is a cutting tip of the composite cutting blade groups, andcutting tips of the composite cutting blade groups are distributed onthe spiral lines.

In some embodiments, a flank surface of the major cutting edge close tothe major cutting edge and a back portion of the major cutting edge awayfrom the major cutting edge are provided at the first step surface; aflank surface of the minor cutting edge close to the minor cutting edgeand a back portion of the minor cutting edge away from the minor cuttingedge are provided at the second step surface.

The major cutting edge, the minor cutting edge and the cutting tip forma composite blade group, and a cutting tooth unit corresponding to thecomposite blade group is a three-sided prism formed by a rake surface,the flank surface of the major cutting edge and the flank surface of theminor cutting edge, wherein the rake surface is arranged on the sidewallof the spiral flute close to the composite blade group.

In some embodiments, each of the composite cutting blade groups iscoaxial with the cone portion; and angles of the composite cutting bladegroups are determined by different linear velocities of the compositecutting blade groups with different diameters.

In some embodiments, a helix angle Ω₀ is an angle of the spiral linesand an axis of the cone portion, and angles of each of the compositecutting blade groups comprise:

an entering angle κ_(r) ranging from 10° to 80°;

an auxiliary angle κ_(r) ranging from 0.5° to 5°;

a normal relief angle α_(n) ranging from 0.5° to 8°;

a normal rake angle γ_(n) ranging from −γ_(n) to +γ_(n), which isdetermined by the flute sidewalls of the spiral flute;

a major cutting edge inclination angle λs ranging from −λ_(s) to +λ_(s),which is determined by the sidewalls of the spiral flute and theentering angle κ_(r);

a minor cutting edge inclination angle λ_(s)′ ranging from >0° to+λ_(s)′, which is determined by the helix angle Ω₀ and the auxiliaryangle κ_(r)′;

an axial major cutting edge inclination angle zz ranging from −λ_(zz) toλ_(zz), which is determined by the position of the sidewalls of thespiral flute (a core thickness of the drill), the helix angle Ω₀ and theentering angle κ_(r);

-   -   an axial minor cutting edge inclination angle λ_(fz) ranging        from >0° to +λ_(fz), which is determined by the helix angle Ω₀        and the auxiliary angle κ_(r)′;    -   an axial relief angle α_(wz) of the major cutting edge ranging        from >0° to +α_(wz), which is determined by the normal relief        angle α_(n) and the auxiliary angle κ_(r)′;

wherein respective cutting tooth units have different normal rake anglesγ_(n), major cutting edge inclination angles λ_(s), minor cutting edgeinclination angles λ_(s)′, axial major cutting edge inclination anglesλ_(zz), axial minor cutting edge inclination angles λ_(fz) and axialrelief angles α_(wz) of the major cutting edge.

In some embodiments, heights of the composite cutting blade groupsarranged on the exterior surface of the cone portion from the front endto the rear end are varied irregularly.

In some embodiments, the top blade comprises a chisel edge, twoauxiliary cutting edges and two straight major cutting edges, and theauxiliary cutting edge is respectively intersected with the straightmajor cutting edge and the chisel edge.

In some embodiments, the cone portion is coaxial with the cylinderportion, and a diameter of a composite cutting blade group at the rearend of the cone portion is equal to a diameter of the cylinder portion;the diameter of the cylinder portion is the drill size of the twistdrill, and a web thickness of the top edge is less than the webthickness of the normal twist drill with the same size.

In some embodiments, the cylinder portion is formed by the spiral fluteand a spiral blade back, and a spiral margin is located in the spiralflute. In some embodiments, formulas for calculating a cross sectionarea Sn of a cutting chip of an n-th composite cutting group and a totalcutting area of the twist drill are as follows:

S1=W1×h1;

Si=Wi×hi;

Sn=Wn×hn;

S (single flute)=51+ . . . +Si+ . . . +Sn;

Sz (double flutes)=2S=2(S1+ . . . +Si+ . . . +Sn)

where Sn is the cross section area of the cutting chip of the n-thcomposite cutting blade group; Wn is the length of the major cuttingedge of the n-th composite cutting blade group; hn is the thickness ofthe cutting chip of the n-th composite cutting blade group; S (singleflute) or Sz (double flutes) is the total cross section area of thecutting chips of the twist drill.

The present invention provides a twist drill, and a cone portion isprovided at a front end of the operating portion, and an exteriorsurface of the operating portion is provided with a spiral flute forshunting cutting chips; the exterior surface of the cone portion isprovided with a plurality of composite cutting blade groups whichsequentially increase in diameter from the front end to the rear end,and the cone portion is provided with the top blade of the drill tip onthe tip. During use, the top edge is used for positioning, and thecutting process is carried out by the top blade and the compositecutting blade groups.

Since diameters of the top edge and the composite cutting blade groupssequentially increase from the front end to the rear end, the cuttingallowance of the object to be cut is reasonably distributed according tothe diameter of the drill hole, and the object to be cut is graduallycut. The cutting force is dispersed on each cutting blade, and thereaction force of each cutting blade is reduced, and fewer burrs aregenerated on the edge of cutting blade, which allows a smoother drillingprocess and a higher drilling efficiency.

The existing twist drill provides two straight main blades which aresymmetrical to process the metal drilling amount in a correspondingsize, which results in a larger power for drilling, and at the sametime, the two cutting blades suffer large reaction forces, so thecutting blades are easily damaged. However, the present inventionovercomes above technical problems.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more clearly illustrate the embodiments of the presentinvention or the technical solutions in the prior art, the accompanyingdrawings of the embodiments or the prior art will be briefly describedbelow. Obviously, the accompanying drawings in the following descriptionare only a part of the embodiments of the present application, and otherdrawings based on the structures shown in the accompanying drawingswithout any creative work can be obtained by those skilled in the art.

FIGS. 1A-1C are schematic diagrams of a normal twist drill in the priorart;

FIG. 2 shows a cutting of a cutting blade of a normal twist drill in theprior art during drilling;

FIG. 3 is a schematic diagram of a twist drill according to anembodiment of the present invention;

FIG. 4A is a top view of a drill tip of the twist drill alone kdirection as indicated by arrow in FIG. 3 according to an embodiment ofthe present invention;

FIG. 4B is a perspective view of a composite blade of the drill tip ofthe twist drill according to an embodiment of the present invention;

FIG. 5A is a schematic diagram of cutting mechanism along K₁ directionas indicated by arrow of FIG. 5B; and FIG. 5B shows the relationshipbetween working faces and cutting angles of a composite blade group of atwist drill according to an embodiment of the present invention;

FIG. 5C is a perspective view of a cutting tooth unit according to anembodiment of the present invention.

FIGS. 6A-6C are close-up views of a top blade of a drill tip of a twistdrill according to an embodiment of the present invention; and

FIG. 7 shows the cutting of a cutting blade of a twist drill accordingto an embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The technical solutions in the embodiments of the present invention areclearly and completely described in the following with reference to theaccompanying drawings. It is obvious that the described embodiments areonly a part of the embodiments of the present invention. Any otherembodiments obtained by the skilled in the art based on the embodimentsof the present invention without departing from the inventive scopeshall fall within the scope of the present invention.

It should be noted that terms such as “up,” “down,” “left,” “right,”“front,” “rear” in the embodiments of the present invention are onlyused to explain the relative position and movement, etc. of components.

In addition, terms “first,” “second,” and the like in the presentinvention are used for description only, and are not intended toindicate or imply their relative importance or the number of technicalfeatures, which explicitly or implicitly indicates that one or morefeatures defined by “first” or “second” may be included in the presentinvention. In addition, the technical solutions of various embodimentsmay be combined to form other embodiments which are considered to fallwithin the claimed scope.

In the present invention, unless specified, terms “connect,” “fix” andthe like should be understood broadly. For example, “fix” may result ina fixed connection, a detachable connection, or an integratedconfiguration of elements. The elements may be connected mechanically orelectrically; or directly connected or indirectly connected through anintermediate medium. Alternatively, two elements may be in communicationor interact with each other unless specified. For the skilled in theart, the specific meanings of the above terms in the present inventioncan be understood according to specific conditions.

As shown in FIGS. 3-7, the embodiment of the present invention providesa twist drill 100 comprising a shank portion 2 and an operation portion1 connected to the shank portion 2. The operation portion 1 comprises acone portion 11 and a cylinder portion 12 which is fixedly connected tothe cone portion 11.

A spiral flute IV for shunting cutting chips is provided on the exteriorsurface of the operation portion 1, and the spiral flute IV is woundfrom the front end of the cone portion 11 to the cylinder portion 12.

A plurality of composite cutting blade groups are provided on theexterior surface of the cone portion 11, and diameters of the compositecutting blade groups sequentially increase from the front end to a rearend of the cone portion, and the composite cutting blade groups eachcomprise a first step surface 141 and a second step surface 142 whichare adjacent. As shown in FIGS. 4A-4B, a top blade II is provided on atip of the cone portion 11, and comprises a chisel edge VII, twoauxiliary cutting edges VIII and two straight major cutting edges 153.

As shown in FIGS. 3, 4A-4B, at the cone portion 11, the cone surfaceintersects with the sidewalls of the spiral flute IV to form a pluralityof first spiral lines 131; at the cylinder portion 12, the cylindersurface intersects with the front sidewall of the spiral flute IV toform a second spiral line 132.

Further, the cylinder portion 12 is formed by a spiral blade back V andthe spiral flute IV, and a spiral margin VI is located on the spiralflute IV and the spiral blade back V.

An intersection of the first step surface 141 and the front flutesidewall of the spiral flute IV is a major cutting edge 143; anintersection of the second step surface 142 and the front flute sidewallof the spiral flute IV is a minor cutting edge 144; and an intersectionof the major cutting edge 143 and the minor cutting edge 144 is acutting tip 1411 of a composite cutting blade group, and cutting tips ofthe composite cutting blade groups are distributed on the spiral lines131. The cutting tips of the composite cutting blade groups aredistributed on two first spiral lines 131, and such distribution iscritical to efficiency, quality, power, etc. of the drilling.

FIG. 4A is a top view (K direction) of the twist drill 100, and FIG. 4Bis a perspective view of the composite cutting blade group.Specifically, as shown in FIGS. 4A-4B, two spiral lines 131 are formed,and the cutting tips of the composite cutting blade groups aredistributed on the two spiral lines 131. Alternatively, three, four orfive spiral lines 131 can also be designed according to the drill coresize of the twist drill 100, which will not be limited by the presentinvention.

As shown in FIG. 3, a flank relief surface 145 of the major cutting edgeclose to the major cutting edge 143 and a back portion 146 of the majorcutting edge away from the major cutting edge 143 are provided at thefirst step surface 141; a flank surface 147 of the minor cutting edgeclose to the minor cutting edge 144 and a back portion 148 of the minorcutting edge away from the minor cutting edge 144 are provided at thesecond step surface 142.

The major cutting edge 143, the minor cutting edge 144 and the cuttingtip 1411 form a composite blade group (shown in FIGS. 5A-5C), and thecutting tooth unit 1410 corresponding to the composite blade group is athree-sided prism formed by a rake surface 133, the flank surface 145 ofthe major cutting edge 143 and the flank surface 147 of the minorcutting edge 144, and the rake surface 133 is a surface of the sidewallin the spiral flute IV for connecting the major cutting edge 143 and theminor cutting edge 144.

A surface on which the back portion 146 of the major cutting edge islocated can be approximately regarded as a conical surface, so the backportion 146 can be considered as a conical major cutting edge back. Theback portion 148 of the minor cutting edge can be approximately regardedas a cylindrical minor cutting edge back. Although the axial minorcutting edge inclination angle is not 0° because the minor cutting edgehas a certain chamfer, a surface on which the back portion 148 of theminor cutting edge is located can be approximately regarded as acylindrical surface.

In some embodiments, each of the composite cutting blade groups iscoaxial with the cone portion 11; angles of the composite cutting bladegroups are determined by different linear velocities of the compositecutting blade groups with different diameters.

The composite cutting blade groups are distributed on the exteriorsurface of the cone portion 11 which is coaxial with the twist drill.The major cutting edge 143 is at an angle (e.g., 45°) to the axis of thecone portion 11, and the minor cutting edge 144 are distributed onexterior surfaces of approximate cylinders which have different radiusand the same axis. The composite cutting blade groups conically andspirally extend along the rake face 133 of the spiral flute IV towardsthe shank (shank portion 2) and terminate at the maximum outer diameterof the twist drill 100. In the composite cutting blade groups, the majorcutting edge and the axis of the cone portion 11 form a relatively smallangle on the conical surface (i.e., the major cutting edge inclinationangle inclination angle of the main blade); the minor cutting edge andthe axis of the cone portion 11 also have a relatively small axial angle(i.e., the minor cutting edge inclination angle) along the axialdirection. The major cutting edge inclination angle and the minorcutting edge inclination angle can be determined by the helix angle ofthe spiral line and the drill core thickness, respectively. Diameters ofthe composite cutting blade groups are diameters of respective segmentsof the cylinder.

In FIG. 3, IV represents the spiral flute; V represents the spiral bladeback; VI represents the spiral margin; Ω₀ represents the helix angle ofthe twist drill; φ represents the peak (top) angle of the twist drillwhich usually ranges from i.e., 118° to 135°.

The angle of a projection of the spiral lines 131 and the axis of thecone portion 11 is a helix angle Ω₀. FIGS. 5A-5C are schematic diagramsshowing a cutting mechanism and a relationship between working faces andcutting angles of a composite blade unit I of a twist drill 100according to an embodiment of the present invention. As shown in FIGS.5A-5C, angles of each of the composite cutting blade units comprise:

-   -   an entering angle κ_(r) ranging from 10° to 80°;    -   an auxiliary angle κ_(r)′ ranging from 0.5° to 5°;    -   a normal relief angle α_(n) ranging from 0.5° to 8°;    -   a normal rake angle γ_(n) ranging from −γ_(n) to +γ_(n), which        is determined by the flute sidewalls of the spiral flute;    -   a major cutting edge inclination angle λ_(s) ranging from −λ_(s)        to −λ_(s), which is determined by the position of the sidewalls        of the spiral flute IV (i.e., core thickness of the drill) and        the entering angle κ_(r);    -   a minor cutting edge inclination angle λs′ ranging from 0°        (exclusive) to λ_(s)′, which is determined by the helix angle Ω₀        and the auxiliary angle κ_(r)′;    -   an axial major cutting edge inclination angle λ_(zz) ranging        from −λ_(zz) to λ_(zz), which is determined by the positions of        the sidewalls of the spiral flute, the helix angle Ω₀ and the        entering angle κ_(r);    -   an axial minor cutting edge inclination angle λ_(fz) ranging        from >0° to +λ_(fz), which is determined by the helix angle Ω₀        and the auxiliary angle κ_(r)′;    -   an axial relief angle α_(wz) of the major cutting edge ranging        from >0° to +α_(wz), which is determined by the normal relief        angle αn, the entering angle κ_(r) and the auxiliary angle        κ_(r)′.

Respective cutting tooth units have different normal rake angles γ_(n),major cutting edge inclination angle λ_(s), minor cutting edgeinclination angle λ_(s)′, axial major cutting edge inclination angleλ_(zz), axial minor cutting edge inclination angle λ_(fz) and axialrelief angles α_(wz) of the major cutting edge.

A composite cutting blade group is also called a composite cutting bladeunit I which comprises a major cutting edge 143, a minor cutting edge144 and a cutting tip 1411. The cutting tooth unit 1410 is a three-sided(rib) prism formed by a rake surface 133 (front sidewall of the twistdrill flute), a flank surface 145 of the major cutting edge and a flanksurface 147 of the minor cutting edge.

The diameters of the composite cutting blade groups sequentiallyincrease from the front end to the rear end of the exterior surface ofthe cone portion 11. Alternatively, in some embodiments, diameters ofthe composite cutting blade groups uniformly or irregularly increase.The diameters of the composite cutting blade groups are designedaccording to the drill core size of the twist drill 100 and the drillsize, the drill depth, the material of work piece, the situation of thetools, etc., which are not limited herein.

Further, heights of the composite blade groups arranged on the exteriorsurface of the cone portion 11 from the front end to the rear end variesuniformly or irregularly. As shown in FIG. 3, l₁, l₂, l₃ . . . l_(n),are heights of respective composite cutting blade groups. The portionwith a length of L₁ is the composite drill tip; the portion with thelength of l_(j) is the drill tip; lengths of l₁, l₂, l₃ . . . l_(n)correspond to the heights of respective composite cutting blade groups.The total length of the cone portion 11 of the twist drill 100 is L₁,and the total length of the twist drill 100 is L. Specifically, l_(n) isgreater than, less than, or equal to l_(n)−1 which can be designedaccording to the required type and property of drilling.

Such cutting tooth has better cutting strength, and is easy to process aproper cutting angle and has a wider application.

FIGS. 6A-6C are close-up views of the top blade II in FIG. 3 of thetwist drill 100. FIG. 6A is a close-up front view of the top blade II inFIG. 3, and FIG. 6B is the close-up left view of the top blade II inFIG. 3, and FIG. 6C is a diagram showing web thickness comparison ofdrill cores. As shown in FIGS. 4A-4B and 6A-6C, the top blade IIcomprises a chisel edge VII, two auxiliary cutting edges VIII and twostraight major cutting edges 153, and the auxiliary cutting edge VIII isrespectively intersected with the straight major cutting edge 153 andthe chisel edge VII, where the chisel edge of the original drill isshortened, so that axial resistance of the drill is reduced duringdrilling.

As shown in FIGS. 4A-4B, the top blade II of the drill tip comprises achisel edge VII, two auxiliary edges VIII and two straight major cuttingedges 153. The composite cutting blade groups with different lengths ofl₁, l₂, l₃ . . . l_(n) extend axially towards the shank portion of thetwist drill 100 along the two first spiral lines 131 (shown in FIGS. 3,4A-4B) to the two spiral margins VI of the cylinder portion 12 of thetwist drill 100, such that an operating portion 1 of the twist drill isformed.

In some embodiments, the cone portion 11 is coaxial with the cylinderportion 12. A diameter of the composite cutting blade group at the rearend of the cone portion 11 is equal to a diameter of the cylinderportion 12, and the diameter of the cylinder portion 12 is the drillsize of the twist drill 100, and the web thickness of the top blade IIis less than the web thickness of the original twist drill. FIG. 6C is adiagram showing the web thickness comparison, where:

K is an original web thickness on the drill tip of the original twistdrill;

K₁ is a web thickness on the drill tip of the twist drill 100;

δ is the included angle of trimmed web thickness.

The standard web thickness of the normal twist drill is determined bythe outer diameter thereof. In case of using a hand tool, when the outerdiameter of the drill bit is large, the twist drill (shown in FIGS.1A-1C) in the prior art, has a large web thickness and a large chiseledge. Thus, during drilling, the axial resistance increases, and thedrilling needs a large drilling force and is time-consuming, and thetwist drill is hard to be positioned.

Therefore, the number of the composite blade groups should be as much aspossible, and the first composite blade group is required to have asmaller diameter. At the same time, the diameter of the first compositeblade group is limited by the web thickness of the drill bit. Therefore,the web thickness of the twist drill should be concern to a standard webthickness. For the twist drill with a larger diameter, when the diameterof the first composite blade group is smaller than the web thicknessthereof, the grinding process for the chisel edge is added to reduce theweb thickness to reduce the cutting force and keep a good positioningperformance. In this embodiment, the drill web thickness K₁ of the topgroup II is much smaller than the original drill web thickness K of thenormal twist drill, and the centering for such drill tip is easy andaccurate.

As shown in FIGS. 1A-1C and 2, the existing twist drill carries out thecutting for a metal with a corresponding size using two symmetricallydistributed straight major cutting edges, where:

-   -   cutting area of a single blade: S1=S2=W×h;    -   total cutting area: S=2S1=2S2=2×W×h.

In some embodiments, the shank portion of the twist drill 100 can be invarious forms, such as a hex shank or round shank or the like. FIG. 7shows the cutting of the cutting blade of the twist drill 100 with theround shank portion during drilling, and the parameters of the top bladeII of the drill tip and the composite cutting blade groups are marked.

Formulas for calculating the cutting area Sn of the n-th compositecutting group and the total cutting area of the twist drill 100 are asfollows:

-   -   S1=Wl×h1;    -   Si=Wi×hi;    -   Sn=Wn×hn;    -   S (single flute)=51+ . . . +Si+ . . . +Sn;    -   Sz (double flutes)=2S=2(S1+ . . . +Si+ . . . +Sn)        where Sn is the cross section area of the cutting chip of the        n-th composite cutting blade group; Wn is the length of the        major cutting edge of the n-th composite cutting blade group; hn        is the thickness of the cutting chip of the n-th composite        cutting blade group; S (single flute) or Sz (double flute) is        the total cross section area of the cutting chip of the twist        drill 100.

A length of the major cutting edge at the drill tip is W1, and the majorcutting edge of the composite cutting blade has a length of Wi, whereW1>Wi; W1 is much smaller than the cutting edge length W of theconventional twist drill, Wi is much smaller than the cutting edgelength W of the conventional twist drill, which disperses the width ofthe cutting layer. The cutting edge thickness of each cutting layer ofthe composite cutting blade is hi, and a cutting thickness hl at thedrill tip is greater than or equal to the thickness h of the cuttinglayer of the conventional twist drill, where hl=hi, which increases theamount of cutting (i.e., the feed rate). A length of the minor cuttingedge at the drill tip is H1, and a length of each cutting edge of thecomposite cutting blade is Hi, where H1>Hi. The minor cutting edge ofthe composite cutting blade has a certain axial auxiliary angle (alsoknown as a side clearance angle), so it only act as to smooth (ratherthan cut) the surface of the machined surface, thereby improve thesurface quality. During manufacturing, the chisel edges of theconventional twist drill (shown in FIGS. 1A-1C) are ground to reduce by80 to 90% to form chisel edges of the twist drill 100 of the presentinvention; and two auxiliary edges (inner edges) are formed, and rakeangles of the inner edges increase from −60°˜10°, which reduces theaxial resistance by more than 50%. Therefore, the axial feed of thetwist drill 100 is quick and the cutting heat at the chisel edge of thedrill tip is greatly reduced.

As the axial force of the twist drill is improved, the centeringaccuracy is also greatly improved, and the composite cutting bladegroups can achieve a multi-stage centering, which greatly improves thesmoothness, roundness and precision of the drilling.

Moreover, the composite cutting blade groups are able to crush chipsinto multiple sections, that is, to crush the removed metal chips tofiner chips which are easy to be removed. Angles of the compositecutting blade groups are reasonably designed according to differentlinear velocities of the composite cutting blade groups with differentdiameters, so the torque resistance is greatly reduced, and thegeneration and gathering of the cutting heat are reduced, and thecutting blade wear is reduced. The uneven wear of the cutting edges withdifferent diameters caused by the different cutting linear speeds isimproved, causing an easier drilling, an increased drilling efficiencyand an extended lifetime of the drill bit.

In summary, a twist drill 100 is illustrated in the embodiment of thepresent invention. A cone portion 11 is provided at a front end of theoperating portion 1, and an exterior surface of the operating portion 1is provided with a spiral flute IV for shunting cutting chips; theexterior surface of the cone portion 11 is provided with a plurality ofcomposite cutting blade groups which sequentially increase in diameterfrom the front end to the rear end, and the cone portion 11 is providedwith the top blade II on the tip. In use, the twist drill is positionedby the top blade II of the drill tip, and the cutting process is carriedout by the top blade II of the drill tip and the composite cutting bladegroups. Since the diameter of the top blade II of the drill tip and thecomposite cutting blade groups sequentially increase from the front endto the rear end, the cutting allowance of the object to be cut isproperly distributed according to the diameter of the drill hole, andthe drilling process needs a small drilling force, and the applieddrilling force is even and appropriate, so that the object to be cut isgradually cut. The cutting force is dispersed on each cutting blade, andthe reaction force of each cutting blade is reduced, so the hand-heldpower tool is stable and can be operated for a long time. Burrs on theedge of cutting blade is reduced, which allows a smoother drilling and ahigher drilling efficiency. The precision of the drilling is guaranteedwhile equipment or personal accidents are avoided. The wear of thecutting blades is uniformly, which prolongs the lifetime of the twistdrill, and reduces unnecessary damages of the tool and the scrapping ofthe workpiece in use. Thus, the processing difficulty and cost arereduced, and the processing efficiency is improved.

The above is only a preferred embodiment of the present invention, whichis not intend to limit the scope of the present invention. Anyequivalent variations based on the content of the specification and thedrawings of the present application shall fall within the scope of thepresent invention.

What is claimed is: 1-8. (canceled)
 9. A method for improving thecentering accuracy of a twist drill when drilling a drill hole in aworkpiece using multistage centering, the method comprising the stepsof: providing a twist drill comprising: a shank portion; an operatingportion axially fixed to the shank portion, the operating portion havinga cone portion axially fixed to a cylinder portion and a plurality ofspiral flutes formed into an exterior surface of the operating portionextending from a front end of the cone portion and at least partway upthe cylinder portion, a spiral line is defined at a top end of a flutesidewall of each of the plurality of spiral flutes, a major cutting edgeis defined at a first intersection of the first step surface and theflute sidewall of each of the plurality of spiral flutes, a minorcutting edge is defined at a second intersection of the second stepsurface and the flute sidewall of each of the plurality of spiral flutesand a cutting tip is defined at a third intersection of the majorcutting edge and the minor cutting edge, and the cutting tip isdistributed on the spiral line of each of the plurality of spiralflutes; a plurality of composite cutting blade groups are provided onthe exterior surface of the cone portion, and diameters of the compositecutting blade groups sequentially increase from the front end to a rearend of the cone portion, and the composite cutting blade groups eachcomprise a first step surface adjacent to a second step surface, a flanksurface of the major cutting edge close to the major cutting edge and aback portion of the major cutting edge away from major cutting edge areprovided at the first step surface, a flank surface of the minor cuttingedge close to the minor cutting edge and a back portion of the minorcutting edge away from the minor cutting edge are provided at the secondstep surface, the major cutting edge, the minor cutting edge and thecutting tip form a composite blade group, and a cutting tooth unitcorresponding to the composite blade group is a three-sided prism formedby a rake surface, the flank surface of the major cutting edge and theflank surface of the minor cutting edge, and the rake surface isarranged on the sidewall of the spiral flute close to the compositeblade group, each of the composite cutting blade groups is coaxial withthe cone portion and angles of the composite cutting blade groups aredetermined by different linear velocities of the composite cutting bladegroups with different diameters; and a top blade is provided on a tip ofthe cone portion; positioning the twist drill atop the workpiece;applying a torque and an axial drilling force to the twist drill;cutting the workpiece with the top blade to create a drill hole in theworkpiece; continuing cutting the workpiece using one or more of theplurality of composite cutting blade groups situated closest to thefront end; and continuing cutting the workpiece using one or more of theplurality of composite cutting blade groups situated closest to thecylinder portion, wherein a cutting allowance of the workpiece isdistributed according to a drill hole diameter so that the workpiece isgradually cut.
 10. The method of claim 9, wherein in the step ofproviding a twist drill, the twist drill further comprises: the topblade comprises a chisel edge, two auxiliary cutting edges and twostraight major cutting edges; the auxiliary cutting edge is respectivelyintersected with the straight major cutting edge and the chisel edge;the cone portion is coaxial with the cylinder portion, and a diameter ofa composite cutting blade group at the rear end of the cone portion isequal to a diameter of the cylinder portion; the diameter of thecylinder portion is the drill size of the twist drill, and a webthickness of the top blade is less than the web thickness of a normaltwist drill; a helix angle ω0 of each of the composite cutting bladegroups is an angle of the spiral lines and an axis of the cone portion;an entering angle κ_(r) of each of the composite cutting blade groupsranges from 10° to 80°; an auxiliary angle κ_(r)′ of each of thecomposite cutting blade groups ranges from 0.5° to 5°; a normal reliefangle α_(n) of each of the composite cutting blade groups ranges from0.5° to 8°; a normal rake angle γ_(n) of each of the composite cuttingblade groups ranges from −γ_(n) to +γ_(n), which is determined by theflute sidewalls of the spiral flute; a major cutting edge inclinationangle λs of each of the composite cutting blade groups ranges from−λ_(s) to +λ_(s), which is determined by the sidewalls of the spiralflute and the entering angle κ_(r); a minor cutting edge inclinationangle λs′ of each of the composite cutting blade groups ranges from >0°to +λ_(s)′, which is determined by the a helix angle ω0 and theauxiliary angle κ_(r)′; an axial major cutting edge inclination angleλ_(zz) of each of the composite cutting blade groups ranges from −λ_(zz)to +λ_(zz), which is determined by the position of the sidewalls of thespiral flute (a core thickness of the drill), the a helix angle ω0 andthe entering angle κ_(r); an axial minor cutting edge inclination angleλ_(fz) of each of the composite cutting blade groups ranges from >0° to+λ_(fz), which is determined by the a helix angle ω0 and the auxiliaryangle κ_(r)′; and an axial relief angle αwz of the major cutting edge ofeach of the composite cutting blade groups ranges from >0° to +αwz,which is determined by the normal relief angle α_(n) and the auxiliaryangle κ_(r)′; wherein respective cutting tooth units have differentnormal rake angles γ_(n), major cutting edge inclination angle λ_(s),minor cutting edge inclination angles λ_(s)′, axial major cutting edgeinclination angles λ_(zz), axial minor cutting edge inclination anglesλ_(fz) and axial relief angles αwz of the major cutting edge; andwherein heights of the composite cutting blade groups arranged on theexterior surface of the cone portion from the front end to the rear endare varied irregularly.
 11. A method for reducing the size of aplurality of metal chips produced when drilling a drill hole in aworkpiece, the method comprising the steps of: providing a twist drillcomprising: a shank portion; an operating portion axially fixed to theshank portion, the operating portion having a cone portion axially fixedto a cylinder portion and a plurality of spiral flutes formed into anexterior surface of the operating portion extending from a front end ofthe cone portion and at least partway up the cylinder portion; and aplurality of composite cutting blade groups are provided on the exteriorsurface of the cone portion, and diameters of the composite cuttingblade groups sequentially increase from the front end to a rear end ofthe cone portion, and the composite cutting blade groups each comprise afirst step surface adjacent to a second step surface; and a top blade isprovided on a tip of the cone portion; positioning the twist drill atopthe workpiece; applying a torque and an axial drilling force to thetwist drill; cutting the workpiece with the top blade to create thedrill hole in the workpiece and thereby producing a plurality of chips;and crushing of the chips by the plurality of composite cutting bladegroups thereby producing a plurality of finer chips, wherein theplurality of finer chips are more easily removed from the drill holethan the plurality of chips via the plurality of spiral flutes.
 12. Themethod of claim 11, wherein in the step of providing a twist drill, thetwist drill further comprises: a spiral line is defined at a top end ofa flute sidewall of each of the plurality of spiral flutes; a majorcutting edge is defined at a first intersection of the first stepsurface and the flute sidewall of each of the plurality of spiralflutes; a minor cutting edge is defined at a second intersection of thesecond step surface and the flute sidewall of each of the plurality ofspiral flutes; and a cutting tip is defined at a third intersection ofthe major cutting edge and the minor cutting edge, the cutting tip isdistributed on the spiral line of each of the plurality of spiralflutes.
 13. The method of claim 12, wherein in the step of providing atwist drill, the twist drill further comprises: a flank surface of themajor cutting edge close to the major cutting edge and a back portion ofthe major cutting edge αway from major cutting edge are provided at thefirst step surface; a flank surface of the minor cutting edge close tothe minor cutting edge and a back portion of the minor cutting edge αwayfrom the minor cutting edge are provided at the second step surface; andthe major cutting edge, the minor cutting edge and the cutting tip forma composite blade group, and a cutting tooth unit corresponding to thecomposite blade group is a three-sided prism formed by a rake surface,the flank surface of the major cutting edge and the flank surface of theminor cutting edge; wherein the rake surface is arranged on the sidewallof the spiral flute close to the composite blade group.
 14. The methodof claim 12, wherein in the step of providing a twist drill, the twistdrill further comprises: each of the composite cutting blade groups iscoaxial with the cone portion; and angles of the composite cutting bladegroups are determined by different linear velocities of the compositecutting blade groups with different diameters.
 15. The method of claim14, wherein in the step of providing a twist drill, the twist drillfurther comprises: a helix angle ω0 of each of the composite cuttingblade groups is an angle of the spiral lines and an axis of the coneportion; an entering angle κ_(r) of each of the composite cutting bladegroups ranges from 10° to 80°; an auxiliary angle κ_(r)′ of each of thecomposite cutting blade groups ranges from 0.5° to 5°; a normal reliefangle α_(n) of each of the composite cutting blade groups ranges from0.5° to 8°; a normal rake angle γ_(n) of each of the composite cuttingblade groups ranges from −γ_(n) to +γ_(n), which is determined by theflute sidewalls of the spiral flute; a major cutting edge inclinationangle λ_(s) of each of the composite cutting blade groups ranges from−λ_(s) to +λ_(s), which is determined by the sidewalls of the spiralflute and the entering angle κ_(r); a minor cutting edge inclinationangle λ_(s)′ of each of the composite cutting blade groups rangesfrom >0° to +λ_(s)′, which is determined by the helix angle ω0 and theauxiliary angle κ_(r)′; an axial major cutting edge inclination angleλ_(zz) of each of the composite cutting blade groups ranges from −λ_(zz)to +λ_(zz), which is determined by the position of the sidewalls of thespiral flute (a core thickness of the drill), the helix angle ω0 and theentering angle κ_(r); an axial minor cutting edge inclination angleλ_(fz) of each of the composite cutting blade groups ranges from >0° to+λ_(fz), which is determined by the helix angle ω0 and the auxiliaryangle κ_(r)′; and an axial relief angle αwz of the major cutting edge ofeach of the composite cutting blade groups ranges from >0° to +αwz,which is determined by the normal relief angle α_(n) and the auxiliaryangle κ_(r)′; wherein respective cutting tooth units have differentnormal rake angles γ_(n), major cutting edge inclination angle λ_(s),minor cutting edge inclination angles λ_(s)′, axial major cutting edgeinclination angles λ_(zz), axial minor cutting edge inclination anglesλ_(fz) and axial relief angles αwz of the major cutting edge.
 16. Themethod of claim 11, in the step of providing a twist drill, whereinheights of the composite cutting blade groups arranged on the exteriorsurface of the cone portion from the front end to the rear end arevaried irregularly.
 17. The method of claim 11, wherein in the step ofproviding a twist drill: the top blade comprises a chisel edge, twoauxiliary cutting edges and two straight major cutting edges; and theauxiliary cutting edge is respectively intersected with the straightmajor cutting edge and the chisel edge.
 18. The method of claim 11,wherein in the step of providing a twist drill: the cone portion iscoaxial with the cylinder portion, and a diameter of a composite cuttingblade group at the rear end of the cone portion is equal to a diameterof the cylinder portion; and the diameter of the cylinder portion is thedrill size of the twist drill, and a web thickness of the top blade isless than the web thickness of a normal twist drill.
 19. A method forimproving the centering accuracy of a twist drill when drilling a drillhole in a workpiece using multistage centering, the method comprisingthe steps of: providing a twist drill comprising: a shank portion; anoperating portion axially fixed to the shank portion, the operatingportion having a cone portion axially fixed to a cylinder portion and aplurality of spiral flutes formed into an exterior surface of theoperating portion extending from a front end of the cone portion and atleast partway up the cylinder portion; and a plurality of compositecutting blade groups are provided on the exterior surface of the coneportion, and diameters of the composite cutting blade groupssequentially increase from the front end to a rear end of the coneportion, and the composite cutting blade groups each comprise a firststep surface adjacent to a second step surface; and a top blade isprovided on a tip of the cone portion; positioning the twist drill atopthe workpiece; applying a torque and an axial drilling force to thetwist drill; cutting the workpiece with the top blade to create a drillhole in the workpiece; continuing cutting the workpiece using one ormore of the plurality of composite cutting blade groups situated closestto the front end; and continuing cutting the workpiece using one or moreof the plurality of composite cutting blade groups situated closest tothe cylinder portion, wherein a cutting allowance of the workpiece isdistributed according to a drill hole diameter so that the workpiece isgradually cut.
 20. The method of claim 19, wherein in the step ofproviding a twist drill, the twist drill further comprises: a spiralline is defined at a top end of a flute sidewall of each of theplurality of spiral flutes; a major cutting edge is defined at a firstintersection of the first step surface and the flute sidewall of each ofthe plurality of spiral flutes; a minor cutting edge is defined at asecond intersection of the second step surface and the flute sidewall ofeach of the plurality of spiral flutes; and a cutting tip is defined ata third intersection of the major cutting edge and the minor cuttingedge, the cutting tip is distributed on the spiral line of each of theplurality of spiral flutes.
 21. The method of claim 20, wherein in thestep of providing a twist drill, the twist drill further comprises: aflank surface of the major cutting edge close to the major cutting edgeand a back portion of the major cutting edge αway from major cuttingedge are provided at the first step surface; a flank surface of theminor cutting edge close to the minor cutting edge and a back portion ofthe minor cutting edge αway from the minor cutting edge are provided atthe second step surface; and the major cutting edge, the minor cuttingedge and the cutting tip form a composite blade group, and a cuttingtooth unit corresponding to the composite blade group is a three-sidedprism formed by a rake surface, the flank surface of the major cuttingedge and the flank surface of the minor cutting edge; wherein the rakesurface is arranged on the sidewall of the spiral flute close to thecomposite blade group.
 22. The method of claim 20, wherein in the stepof providing a twist drill, the twist drill further comprises: each ofthe composite cutting blade groups is coaxial with the cone portion; andangles of the composite cutting blade groups are determined by differentlinear velocities of the composite cutting blade groups with differentdiameters.
 23. The method of claim 22, wherein in the step of providinga twist drill, the twist drill further comprises: a helix angle ω0 ofeach of the composite cutting blade groups is an angle of the spirallines and an axis of the cone portion; an entering angle κ_(r) of eachof the composite cutting blade groups ranges from 10° to 80°; anauxiliary angle κ_(r)′ of each of the composite cutting blade groupsranges from 0.5° to 5°; a normal relief angle α_(n) of each of thecomposite cutting blade groups ranges from 0.5° to 8′; a normal rakeangle γ_(n) of each of the composite cutting blade groups ranges from−γ_(n) to +γ_(n), which is determined by the flute sidewalls of thespiral flute; a major cutting edge inclination angle λ_(s) of each ofthe composite cutting blade groups ranges from −λ_(s) to +λ_(s), whichis determined by the sidewalls of the spiral flute and the enteringangle κ_(r); a minor cutting edge inclination angle λ_(s)′ of each ofthe composite cutting blade groups ranges from >0° to +λ_(s)′, which isdetermined by the helix angle ω0 and the auxiliary angle κ_(r)′; anaxial major cutting edge inclination angle λ_(zz) of each of thecomposite cutting blade groups ranges from −λ_(zz) to +λ_(zz), which isdetermined by the position of the sidewalls of the spiral flute (a corethickness of the drill), the helix angle ω0 and the entering angleκ_(r); an axial minor cutting edge inclination angle λ_(fz) of each ofthe composite cutting blade groups ranges from >0° to +λ_(fz), which isdetermined by the helix angle ω0 and the auxiliary angle κ_(r)′; and anaxial relief angle αwz of the major cutting edge of each of thecomposite cutting blade groups ranges from >0° to +αwz, which isdetermined by the normal relief angle αn and the auxiliary angle κ_(r)′;wherein respective cutting tooth units have different normal rake anglesγ_(n), major cutting edge inclination angle λ_(s), minor cutting edgeinclination angles λ_(s)′, axial major cutting edge inclination anglesλ_(zz), axial minor cutting edge inclination angles λ_(fz) and axialrelief angles αwz of the major cutting edge.
 24. The method of claim 19,in the step of providing a twist drill, wherein heights of the compositecutting blade groups arranged on the exterior surface of the coneportion from the front end to the rear end are varied irregularly. 25.The method of claim 19, wherein in the step of providing a twist drill:the top blade comprises a chisel edge, two auxiliary cutting edges andtwo straight major cutting edges; and the auxiliary cutting edge isrespectively intersected with the straight major cutting edge and thechisel edge.
 26. The method of claim 19, wherein in the step ofproviding a twist drill: the cone portion is coaxial with the cylinderportion, and a diameter of a composite cutting blade group at the rearend of the cone portion is equal to a diameter of the cylinder portion;and the diameter of the cylinder portion is the drill size of the twistdrill, and a web thickness of the top blade is less than the webthickness of a normal twist drill.